A Heat Pump Liquid Heater (“HPLH”) uses a refrigeration system to extract heat from the surrounding environment to heat a liquid. An HPLH system typically consumes less than half the energy relative to a conventional, electric-resistance or gas burner liquid heater. An HPLH system is based on a reverse refrigeration cycle with the HPLH system using an electrical compressor to compress the refrigerant to a liquid state which is at a high pressure and temperature. The refrigerant at high temperature and pressure transfers heat to a liquid and then goes though an expansion process where the temperature and pressure of the refrigerant are reduced to form a low temperature refrigerant. The low temperature refrigerant passes through an evaporator absorbing heat from the surrounding air and converts into a gaseous state. The gaseous refrigerant is re-compressed in the compressor and the aforementioned process continues. The liquid is heated by both the heat transferred from the ambient air and the electricity used to operate the compressor making HPLH more than 100% efficient.
There are basic two types of HPLH systems described in the references: (i) integrated with a liquid tank, and (ii) a standalone without a liquid tank. In both types of systems, the condenser coil is either immersed into the liquid in the tank where a pump is not required or the liquid is pumped from the tank to the heat pump condenser coil. HPLH systems are attached with backup heating system like electrical elements or gas heaters if the HPLH system fails, has reduced performance levels, or if the demand for hot liquid surpasses the capacity of the HPLH system.
An HPLH system with a liquid pump adds to costs, complexity, and maintenance. It also reduces efficiency because pumps need additional electrical energy to run. An integrated HPLH system with a tank is limited to serve only new constructions or to replace old water heater tanks. There are a few references of standalone HPLH systems without using liquid pumps (see, for example, U.S. Pat. Nos. 5,946,927 and 6,233,958). U.S. Pat. No. 5,946,927 discloses a condenser coil rapped around the water tank on the outside for heat transfer to the liquid inside the tank. Such a system has a high cost of manufacturing and reduces the efficiency of heat transfer from the condenser coil to the liquid because the condenser coil is not in direct contact with the liquid.
U.S. Pat. No. 6,233,958 describes a standalone heat pump water heater for residential use with a condenser assembly having a tube-in-tube cylinder configuration such that an outer cylinder carries a superheated refrigerant and an inner cylinder returns the refrigerant to an expansion process. The condenser assembly is inserted into the water tank though an existing opening in the top of the tank. As the refrigerant condenses along the interior surface of the outer cylinder, the heat from the refrigerant is transferred to the water. This heat pump water heater has a limitation of heat transfer to the water because of the limited surface area provided by the cylindrical condenser exposed to the water. The limited heat transfer reduces the efficiency of the system. The diameter of the outer cylinder should be smaller than the size of the opening in the tank. The typical size of the opening in residential water tanks is about ¾ inches and the height of tank is typically about 3-5 ft. Therefore, the maximum heat transfer area is limited by these dimensions. Another limitation of the heat pump water heater is the reduction in efficiency due to heating the returning refrigerant by entering hot refrigerant. The heat gained in the return refrigerant is wasted. Such a heat pump water heater is typically suited for a low capacity compressor but will significantly reduce the efficiency of a typical residential heat pump water heater.
Therefore, there is a need for efficient HPLH systems where liquids such as water can be heated in a shorter period of time while reducing the amount of energy used to heat the liquid. Further, there is a need for HPLH systems that can be easily installed or easily retrofitted onto preexisting liquid tanks while providing reduced heating times along with reduced power consumption. There is also a need for HPLH systems with increased efficiencies that further provide reduced costs related to materials and installation of the HPLH systems.